The present application relates to navigation systems. The invention finds particular application in aircraft landing systems which provide precision elevation guidance to a user, such as a controller or pilot during approach and landing.
Various precision aircraft landing systems have been employed to assist a pilot in maintaining a desired glide path to a runway. The Instrument Landing System (ILS) is commonly used for precision approaches; however, ILS systems are prone to interference from nearby FM broadcasts, require extensive terrain grading and property acquisition at some airport sites and are vulnerable to guidance beam distortion when considering construction near an airport. The Microwave Landing System (MLS) is much less commonly employed than ILS, and is being phased out in response to economic concerns. Precision Approach Radar (PAR) are commonly used in military environments and require a ground operator to verbally convey glide path guidance corrections to the pilot via a communications link. Global Positioning System based landing aids have been proposed which include two systems under development, the Wide Area Augmentation System (WAAS) and the Local Area Augmentation System (LAAS) both of which are subject to jamming and spoofing, and may not be suitable for sole means precision approach.
Aircraft navigation systems which employ the Air Traffic Control Radar Beacon System (ATCRBS) transponder are generally known in the art. Transponders are typically deployed on aircraft to facilitate the Secondary Surveillance Radar (SSR) function of monitoring and controlling enroute aircraft. Most commercial aircraft are equipped with two transponder antennas, one on the top and another on the bottom of the aircraft""s fuselage to maintain reliable transponder replies during aircraft turns. Such transponder antenna configurations are known as diversity antennas. A transponder equipped with diversity antennas selects the antenna which received the highest amplitude interrogation signal from a ground station to transmit the coded reply message. International Standards and Recommended Practices presently require that the horizontal distance between the top and the bottom antennas be less than 7.6 meters, in order to control the apparent SSR range jitter from reply to reply due to antenna diversity switching. The vertical separation of diversity antennas varies as a function of aircraft fuselage height and can be approximately between 3 and 10 meters.
Landing systems which use the ATCRBS transponder must determine the aircraft""s position, compare it to a desired approach path, and transmit any required correction to the aircraft. Nehama U.S. Pat. No. 3,564,543 describes such a system, which uses symmetry and simplified mathematics to define a conical approach path. In general, the position determining system disclosed in Nehama and like systems is based on transponder reply time-of-arrival measurements derived from the time required for the interrogation to travel to the transponder, for the time for transponder to respond, and the time required for signals to travel between the landing aircraft and a plurality of locations on the ground. From these distances, the aircraft""s position is estimated. The Nehama patent acknowledges the existence of variable transponder reply time which can induce substantial errors in the navigation solution. As a compromise, Nehama arranges the transmitter and sensors in a substantially vertical geometric plane transverse to the length of the runway. This arrangement projects the error in a horizontal direction along the axis of the runway. As a side effect, this arrangement requires the use of elevated antenna towers in the vicinity of the airport, for if all the sensors were positioned at ground level, and thus in a horizontal plane, the calculated altitude of the aircraft would contain substantial errors, which would be impermissible for a precision landing system.
Stoltz U.S. Pat. No. 5,017,930 discloses a system which advances over Nehama by, among other things, also solving for transponder encoding delay by employing four sensors. Unfortunately, the time of arrival measurements used by landing systems such as that described in Nehama and Stoltz are subject to significant multipath errors. These multipath errors are induced by terrain features along the approach path to the runway and induce errors in the time of arrival measurements. Errant time of arrival measurements degrade the navigational solution, and thus reduce the accuracy of guidance signals transmitted to the aircraft.
It is desirable for landing systems to comply with the International Standards and Recommended Practices limit on the excursion characteristics of the navigation on-path signal which includes bends, scalloping, roughness and other aberrations with a two-sigma limit roughly equivalent to 3 meters at a point 1.4 km from the glide path Point of Runway Intercept. Unfortunately, diversity antenna switching, even on the smallest aircraft, can potentially cause performance out of this window.
The present invention contemplates an improved method and apparatus which overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, a method of determining a position of an aircraft having a transponder which transmits a reply signal in response to an interrogation signal includes first receiving reply signals on a plurality of antennas disposed as a vertically oriented array. Characteristics of the reply signal, such as differential phase, amplitude, frequency and the like, are measured and used to estimate the aircraft position. The differential phase is analyzed between at least two reply signals to determine whether respective reply signals originate from different antennas on the aircraft. In the event the reply signals are determined to originate from diversity antennas, the estimated position is adjusted to compensate for the distance between the respective antennas. The method also can calculate an error between the adjusted position and a desired position and convey this error to a user such as a pilot, air traffic controller, or to cockpit displays of other aircraft.
A precision aircraft landing system determines on a real-time basis the location of an aircraft by measuring elapsed time between interrogation and transponder reply signal at a plurality of predetermined locations. The system manages the effects of multipath and achieves accurate aircraft positioning by measuring the transponder reply differential phase to compute angle-of-arrival.
The present invention has the capability to compensate for transponder diversity antenna switching, and as a consequence of this compensation, achieve an elevation estimate with the least dynamic lag.
In accordance with another aspect of the present invention, a multipath correction is applied to the selected characteristics to compensate for multipath errors induced in the estimated position, thereby achieving the best possible detection and compensation for diversity antenna.
One advantage of the present invention resides in the ability to precisely determine aircraft position based on a cooperative transponder reply signal originating from an aircraft.
Another advantage of the present invention resides in the ability to manage or cancel the effects of multipath returns of the transponder reply signal.
Another advantage of the present invention resides in the ability to accurately determine aircraft position by measuring both transponder reply angle of arrival and time of arrival.
Still further advantages will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.